Share this story

“The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire… I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few metres… When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops.”

This eyewitness account doesn’t describe the detonation of a nuclear bomb. The events occurred in 1908, long before the Manhattan Project. The account describes the Tunguska Event, an enigmatic explosion in central Siberia thought to be caused by a comet or meteoroid. It must have exploded before reaching the ground, unleashing the energy of a thousand Hiroshima bombs.

The blast flattened over 2,000 square kilometers of Siberian forest, though the trees at ground zero were left standing, stripped of their branches and charred. Bright night skies and strange sunsets were reported across central Asia and northern Europe for several days. Due to its extremely secluded location, it would be nearly two decades before an expedition examined the destruction.

What they found was baffling—all the signs of a major impact, but no crater and not a single piece of an impactor. L.A. Kulick, the Russian mineralogist who led the first expedition, undertook massive efforts to dig for fragments, but his effort was never rewarded. Consequently, it was thought that the impactor, possibly an icy comet, had utterly vaporized in the explosion, leaving no trace.

But the comet hypothesis has some issues. Researchers examining the peaty sediment have found slight anomalies of iridium and isotopes of hydrogen, carbon, and nitrogen suggestive of chondritic (stony) asteroids. Others have also found meteorite-like microparticles in the remains of trees there. Still, there’s no smoking gun.

This week in the journal Geochemistry, Geophysics, Geosystems (or G3), a team of Italian researchers reports that they’ve found tantalizing evidence of what could be a stony chunk of meteorite beneath a small lake eight kilometers from the epicenter of the blast.

The team started looking at Lake Cheko over a decade ago. This oblong, 700 meter long lake looks a bit strange for a thermokarst lake. The bottom of the lake is funnel shaped, with a maximum depth of 50 meters. Whether Lake Cheko was created during the Tunguska Event has been a matter of debate for some time.

In 1999, the Italian group took sediment cores from the lake and used seismic energy to image the sediment beneath the lake. (The technique is similar to a submarine’s sonar, but can penetrate below ground.) They found that while the upper meter of sediment is nicely layered clay with preserved plant material—standard fare for swampy lakes—the sediment below this is chaotic and sandy, with very little organic matter. They estimated that the clay-rich lake sediment had been accumulating for about 100 years, which gives a start date of around the time of the Tunguska Event.

The seismic imaging clearly identified these layers, but also found something odd below the deepest point of the lake. Seismic imaging doesn’t give you detailed pictures like sonograms, which can let you count a baby’s fingers, but it did indicate a small area of higher density.

The team returned in 2009 for a detailed magnetic survey. Rather than actively emitting energy waves that bounce off objects and return to a sensor, magnetometers passively measure magnetic field strength very precisely. This allows them to detect variations caused by materials with different magnetic properties. By making regular measurements in a tight grid across the lake’s surface, they detected an anomaly (albeit weak) in the same area where the seismic imaging indicated something dense.

The researchers say this is all consistent with their hypothesis about Lake Cheko—that it is an impact crater that may still be home to a fragment of the body that caused the Tunguska Event. This isn’t the first time they’ve published the idea, however, and some researchers find it highly unlikely. They’ve argued that the shape of the lake bottom does not fit with other impact craters, and that the signal in the seismic data is more likely a sedimentary feature. In addition, they contend that an impact crater of that size would be extremely unusual for a stony meteorite, which isn't nearly as strong as iron meteorites.

The Italian researchers say that the sediment at Tunguska—swampy peat and permafrost—makes for unusual impact conditions. The fragments could have penetrated some distance into the ground before causing the explosion of sediment. And, obviously, they think the magnetic evidence bolsters their claim about Lake Cheko. Their paper concludes, “These mutually independent and consistent observations are compatible with the presence of an asteroid or a compacted and heated layer that could have been generated during the impact and is now buried below ≈10 m of [lake sediment] at the lake center.”